Laser beam welding is a joining technique that offers a number of advantages over conventional gas metal arc welding (GMAW), such as for instance low heat input, short cycle time and good cosmetic welds. The process is frequently used in high volume applications, such as for instance the welding together of powertrain components in the automotive industry. In this way, components that contain multiple grades or multiple types of material can be manufactured at relatively low production cost. By way of a specific and non-limiting example, a flexplate contains a ring gear that is welded to an outer rim of a thin disc of stamped steel. The main function of the flexplate in an automobile is to connect the transmission's torque converter to the engine's crankshaft. A flexplate is used instead of a solid, non-flexing disc because the torque converter's outer metal shell tends to expand with heat under continuous operation and its flexing feature prevents it from cracking and failing prematurely. The ring gear is typically treated by carbonization to increase the hardness and wear-resistance of its teeth, while the stamped disc, usually large in diameter, is made of low carbon steel. In order to enhance the wear-resistance on the surface, nitriding of the surface of the disc is typically required by original equipment manufacturers (OEMs).
Unfortunately, laser beam welding of nitride steel components typically produces low strength welds that are also highly porous. These characteristics make laser welds generally unsatisfactory for powertrain applications that involve the joining together of nitrided components. The difficulty lies in the fact that laser beam welding is a deep penetration welding process, and that the nitride layer extends deep inside the weld joint. When the nitride layer is melted inside the joint during laser beam welding, nitrogen is released into and is retained within the resulting weld pool. Subsequently, the nitrogen gas coalesces to form bubbles as the weld pool solidifies. Since laser beam welding is a fast process there is insufficient time for the nitrogen bubbles to escape out of the weld pool under normal welding conditions, and as a result the bubbles of nitrogen gas become occluded in the weld material, thereby increasing porosity and reducing the strength of the resulting laser weld.
Past attempts to improve the quality of laser welds in nitrided components have focused on optimizing the welding parameters and modifying the characteristics of the laser beam. Overall, these attempts have failed to achieve satisfactory results. Currently, the only process that is known to produce satisfactory laser welds in nitrided components requires the removal of the nitride layer along the joint prior to performing laser beam welding. However, such an approach is not a practical solution for high production-volume applications.
It would therefore be beneficial to provide a method for laser beam welding of nitride steel components that overcomes at least some of the above-mentioned limitations and disadvantages of the prior art.